Coal Mine ConnectionsCoal Mine Connections

Lessons from ExperienceLessons from Experience

Why are we talking about Coal Why are we talking about Coal mine connectionsmine connections

Because we had a situation where the Because we had a situation where the customer wanted a service that was out of customer wanted a service that was out of our standards and therefore caused our standards and therefore caused confusion and eventually a problem.confusion and eventually a problem.

Here was the planHere was the plan

1.47 mi

LIVINGSTON

PODISH

0.13

mi

0.90 mi

0.50

mi

SUMMIT

JCT.

3.15 mi

ELLSWORTH

CALVIN

JCT.

1.30 mi

0.69

mi

WEIRN.O.

LITTLE

A

25 kVANDREWS

SHAFT

RICHARDSON

0.19

mi

4.71 mi

556.5

25 kV

0.01

mi

3/0

AA

AC

556.

5

3/0

AC

SR

4/0,

1/0

4/0 CU,1/0 CU

LONGFARMSHAFT

0.17

mi

3/0

AC

SR

3/0 ACSR3/0 ACSR

3/0

AC

SR

0.60

mi

3/0

AC

SR

100

ft.

1/0

AL

U

LONG FARM BLEEDER SUBSTATION

U

100

ft.

3/0

AC

SR

CONSTRUCT 100 FEET OF 1/0 AL,UNDERGROUND 25 kV LINE

INSTALL A 25 kV AIR SWITCH AT TAP

INSTALL A 1.5 MVA, 25-4.16 kV PAD-MOUNTEDTRANSFORMER PACKAGE WITH 4 kV METERING 1.5 MVA,

25-4.16 kVPAD-MOUNT

LFB54173

LONG FARM BLEEDER

TAP

LFBT53773

LSMJ-EPJ (400)

What HappenedWhat Happened

The transformer was energized with no load The transformer was energized with no load and the low side arrester failed. The high and the low side arrester failed. The high side fuse failed and launched upwards and side fuse failed and launched upwards and stuck into the roof of the transformer.stuck into the roof of the transformer.

Blown Arrester

Neutral ungrounded

3 CT’s

2 VT’sHigh Side VT’s Grounded

The situation developed because

1. Planning not aware that coal mine wanted ungrounded service at 4160. They were told that this customer want 4160 service nothing was said about ungrounded service

2. Planning issued a plan for 4 wire service

3. Pad mount transformer was shipped with arresters installed, this is not what the T&D manual wants

4. 3 element metering specified, but only 2 VT’s showed up

5. It was a rush job so a decision was made to use 2.5 element metering

6. When the transformer was ready to be energized, the Coal company inspector told our people to remove the ground

7. Our crew was confused and called their boss and he checked

8. The metering people said they would have to change the metering

9. Our crew was told to disconnect the metering, they did by opening the low side metering switches

10. They were instructed to remove the ground

11. They were told to go ahead and energize

12. Arrester failed causing a phase to phase fault and failed the high side transformer fuse

Metering opened

Why did it happenWhy did it happen

Here is the Schematic of how everything Here is the Schematic of how everything was hooked up when energized.was hooked up when energized.

No Ground on Transformer

High side on metering VT’s Grounded for 2.5 element metering

No Ground on Transformer

High side on metering VT’s Grounded for 2.5 element metering

This arrester saw a voltage of 1.5p.u. and failed, when it failed it blew stuff all over the place. This stuff caused a phase to phase fault in the transformer, also when the arrester failed, it would fail in the shorted mode, causing line to line voltage across the VT’s putting them into saturation, which would cause a high current to flow through them

This transformer used Cooper fuses which was an old design as it did not provide positive latching, using the right hand rule the fuse could have been forced out of the holder and the expelled gas would have launch it upwards into the lid. As it would see maximum fault current at this point.

So what do coals mines want?So what do coals mines want?

There are two types of mines we encounter There are two types of mines we encounter on our system.on our system.

Above Ground mines, I like to call them Above Ground mines, I like to call them strip mines.strip mines.

Below ground mines such as QuecreekBelow ground mines such as Quecreek

Above ground mines usually require our Above ground mines usually require our standard service, but below ground mines standard service, but below ground mines are different.are different.

Both above ground mining and below Both above ground mining and below ground mining are govern by the State ground mining are govern by the State Mining Laws. Each State has their own Mining Laws. Each State has their own group that enforces these laws.group that enforces these laws.

www.dep.state.pa.us/dep/deputate/minres/dwww.dep.state.pa.us/dep/deputate/minres/dms/guidelines/ms/guidelines/

Below ground mining operations Below ground mining operations requires special standards from what we requires special standards from what we

are use toare use to Why is below ground so special. Underground Why is below ground so special. Underground

mining deals with methane gas and water. Two mining deals with methane gas and water. Two things that don’t mix well with electric.things that don’t mix well with electric.

For below ground mining there are two For below ground mining there are two componentscomponents

a. Electric that goes underground for operating a. Electric that goes underground for operating equipmentequipment

b. An above ground fan that pushes air b. An above ground fan that pushes air underground for ventilationunderground for ventilation

Lets look at each componentLets look at each component

Above ground air fan.Above ground air fan. If that fan goes off line then the mine must be If that fan goes off line then the mine must be

evacuated immediately. Not easy when you are a evacuated immediately. Not easy when you are a mile or so into a mine.mile or so into a mine.

The operators would like to keep the fan operating The operators would like to keep the fan operating until the end of a shift when the mine is emptied.until the end of a shift when the mine is emptied.

If you have a line to ground fault with a solidly If you have a line to ground fault with a solidly grounded neutral the circuit protection will trip the grounded neutral the circuit protection will trip the fan immediately. But if the fan were to be served fan immediately. But if the fan were to be served from a delta connection then you can have a line to from a delta connection then you can have a line to ground fault and have time to look for the trouble.ground fault and have time to look for the trouble.

Therefore, most operators would like to Therefore, most operators would like to serve there fan from a delta system, so a serve there fan from a delta system, so a line to ground fault does not cause the fan line to ground fault does not cause the fan to shut down and can be corrected not to shut down and can be corrected not immediately, but in a timely manner.immediately, but in a timely manner.

DEP recognizes this and allows either the DEP recognizes this and allows either the fan to be served from a solidly grounded fan to be served from a solidly grounded wye or from a delta connection.wye or from a delta connection.

Most of the fans are in the hundreds of Most of the fans are in the hundreds of horsepower so it requires voltages to horsepower so it requires voltages to operate at 4160 or 2400. operate at 4160 or 2400.

If the mine has a 4160 volt motor they If the mine has a 4160 volt motor they will want to take voltage at 4160 delta.will want to take voltage at 4160 delta.

To give them this delta connection To give them this delta connection requires us to operate the transformer as requires us to operate the transformer as an ungrounded wye.an ungrounded wye.

We have transformers that can be We have transformers that can be connected either 4160 wye or 2400 volts connected either 4160 wye or 2400 volts delta.delta.

However this delta connection is a non However this delta connection is a non standard for us at 4160 volts, so we have standard for us at 4160 volts, so we have to be careful on how we hook it up and the to be careful on how we hook it up and the equipment connected to it.equipment connected to it.

So what does the mines want for So what does the mines want for the surface fanthe surface fan

No ground

Metering 3 wire

Coal mine owns

or

DEP wants this connection between ground grids as they require 4 ohms or less

AP Transformer

Fan and other stuff

So what does the mines want for So what does the mines want for the surface fanthe surface fan

No ground

Metering 3 wire

Coal mine owns

or

DEP wants this connection between ground grids as they require 4 ohms or less

AP Transformer

Fan and other stuff

10 foot min

If arresters are installed they have to be phase to phase rated

So what does the mines want for So what does the mines want for the surface fanthe surface fan

No ground

Metering 3 wire contained in the transformer

Coal mine owns

or

AP Transformer Fan and other stuff

10 foot min

If arresters are installed they have to be phase to phase ratedAs a cost savings for

both us and the coal mine we are using padmounts

They would like us to have a ground ring installed around the padmount with 2 ground rods as a min. If the coal company ever buys the padmount they will have to have a 4 ohms or less resistance.

So what does the mines want for the So what does the mines want for the

deep mine servicedeep mine service

No ground

Metering 3 wire

Coal mine owns

or

DEP wants this connection between ground grids as they require 4 ohms or less

AP Transformer

Zig-Zag Transformer if being served Delta and a grounding resistor

10 foot minIf arresters are installed they have to be phase to phase rated

Grounding resistor to limit fault to 25 amps

Ground Bed <4 ohms

Zig-zag if delta

Neutral ground bed <4 ohms 25ft away, but no more than 100ft from transformer

Insulated neutral

Transformer connections and Transformer connections and voltagevoltage

The highest voltage allowed is 15kV to The highest voltage allowed is 15kV to serve the coal mineserve the coal mine

Delta wye, delta delta, wye delta are Delta wye, delta delta, wye delta are permitted provided a grounding resistor and permitted provided a grounding resistor and a zig zag on delta connections to provide a zig zag on delta connections to provide phase to ground faults limited to 25 ampsphase to ground faults limited to 25 amps

A Wye Wye is not permitted A Wye Wye is not permitted

DEP requires 4 ohms for the ground grid or DEP requires 4 ohms for the ground grid or

less instead of 5 ohms as some standards less instead of 5 ohms as some standards state, because they want to limit the voltage state, because they want to limit the voltage developed to 100 volts at 25 amps through developed to 100 volts at 25 amps through the neutral resister, so that makes the the neutral resister, so that makes the resistance 4 ohms or less.resistance 4 ohms or less.

The isolated ground grid has to be a The isolated ground grid has to be a minimum of 25 feet away from the power minimum of 25 feet away from the power transformer, but no more than 100 ft from transformer, but no more than 100 ft from the power transformer per the federal the power transformer per the federal mining laws.mining laws.

RememberRemember

Any time the low side is not solidly Any time the low side is not solidly groundedgrounded

A. Phase to Phase Arresters must be usedA. Phase to Phase Arresters must be used B. 3 wire metering must be used.B. 3 wire metering must be used. When you do not solidly ground the low When you do not solidly ground the low

side, line to ground loads can not be served. side, line to ground loads can not be served. So station service must be a phase to phase So station service must be a phase to phase connected load.connected load.

What is 5S, 6S, 9S meteringWhat is 5S, 6S, 9S metering

5S or two element metering is for 3 wire 5S or two element metering is for 3 wire systems ie cannot serve line to ground loadssystems ie cannot serve line to ground loads

6S or 2 ½ element metering is for 4 wire 6S or 2 ½ element metering is for 4 wire systems. Not used as a standard any more.systems. Not used as a standard any more.

9S or 3 element metering is for 4 wire 9S or 3 element metering is for 4 wire systemssystems

3 element, 2 VT’s, 2 CT’s

2 ½ element, 2 VT’s, 3CT’s

3 element, 3 VT’s, 3CT’sVT’s connected to ground

VT’s connected to ground

So what was the problem

Neutral not connected

2 ½ element metering used

2.5 element metering caused

neutral to move here

Arrester connected line to neutral was not rated for 1.5 p.u. overvoltage there fore boom

The situation developed because

1. Planning not aware that coal mine wanted ungrounded service at 4160. They were told that this customer want 4160 service nothing was said about ungrounded service

2. Planning issued a plan for 4 wire service

3. Pad mount transformer was shipped with arresters installed, this is not what the T&D manual wants

4. 3 element metering specified, but only 2 VT’s showed up

5. It was a rush job so a decision was made to use 2.5 element metering

6. When the transformer was ready to be energized, the Coal company inspector told our people to remove the ground

7. Our crew was confused and called their boss and he checked

8. The metering people said they would have to change the metering

9. Our crew was told to disconnect the metering, they did by opening the low side metering switches

10. They were instructed to remove the ground

11. They were told to go ahead and energize

12. Arrester failed causing a phase to phase fault and failed the high side transformer fuse

What can be done in the future to try to prevent these occurrences

On all plans specify whether the service is Delta, Wye, or Grounded Wye.

Specify the voltage, both phase to phase and phase to neutral where appropriate.

Create a single line if it is something outside of our standards.

Then if something does fit the plan or changes in the field do not fit the plan, the job should be stopped until it is reviewed by planning and engineering.